November 12, 2006 Bonus Bonus Blog
It appears that which side you mount a horse (at least historically) may come down to whether you carried a curved sword (Arab), or a straight sword (Viking, English). Meanwhile:

“Cisco and Pancho mounted from the left. Of course they were American Movie Mexicans. Is it possible people mount whichever way they do to show their political preferences?"
—Steve Lodge

No, However, Steve found this discussion on left-hand mounting via Google:

As The Stirrup Turns
“I must agree with Bowyer that most horses are biased to the left. However, I would dispute the suggestion that mounting from the left is connected to this bias. Horses (in the Western European tradition) are mounted from the left as a result of a closed feedback loop powered by training and tradition. Most riders expect to mount from the left and so the young horse is taught that its rider will mount from the left. Because most horses expect to be mounted from the left, beginner riders are taught to mount from the left, and so on.

“The completely inexperienced horse has no preference for the rider mounting on either side, it is only habit which makes him prefer the left approach. I would always advise someone who owns a horse to teach it to stand to be mounted from the 'wrong' side occasionally. This will ensure that neither the horse nor the rider will face undue problems if it should be necessary to do such a thing in an emergency.

“Historically, I suspect that before the invention of the stirrup most horses were mounted from the right, it being easier for a right-handed person to get a good grip on the mane or neck to help them vault on from that side. With the discovery of stirrups tradition diverged.

“In those countries where armed men (the most numerous class to ride horses) wore short, curved swords belted high on their waist, mounting continued to be from the right. The Arabs mounted from the right and spread that tendency across north Africa and into Spain; from Spain it spread to the New World and Native Americans mounted from the right.

“In Christian Europe the most usual knight's weapon was a long straight sword. Mounting from the right with a long straight sword hanging by your left side is, at best, awkward. Mounting from the left made far more practical sense. From Europe this tradition spread to northeastern America, here white Americans mount from the left.“
—Judith Seaman

“If one always gets on and off at the left side of a horse, it becomes natural to mount from the verge with the horse on the left side of the road. This done, it will be natural to move off on the left, a tendency possibly reinforced by the horse's left bias.

“If meeting a hostile horseman coming the other way, a right-handed rider would surely want to be on the left of the road so as to deploy his weapons on the right, not across his own horse. It thus seems strange that so many countries now ride and drive on the right. Were the rulers of Continental Europe more concerned about the ability of their cavalries to dominate unmounted people at the road-side? Or was it an extension of the rules that applied at sea?

“Turning left may not be just a tendency of the horse. I believe that at one stage in the Second World War, British fighter pilots were instructed to vary the direction in which they turned when attacked, since the Luftwaffe had noticed that most pilots turned left under stress, and had devised tactics to take advantage of this. Was it a universal tendency, or just a result of being brought up with Britain's rule of the road?”
—Alan Self

“Personally I mount them from the back.”
—Tom Mix and Johnny Wadd